1. Field of the Invention
The present invention relates to a blood information measuring method and apparatus for measuring blood information from an image signal of a blood vessel.
2. Description Related to the Prior Art
Endoscopes are widely used for observation of a lesion located inside a human body. The endoscope is provided with an insert section to be introduced into the human body, and a handling section for steering the insert section. The insert section has a lighting window and an imaging window at its distal end. An internal body portion is imaged through the imaging window while being illuminated with light through the lighting window, and an obtained endoscopic image is displayed on a monitor.
As a light source of the endoscope, a white light source such as a xenon lamp or a metal halide lamp is conventionally available. Additionally, there is a method recently in the limelight in which light (narrow band light) of a narrow wavelength band is used as illumination light to facilitate finding out the lesion (refer to US Patent Application Publication No. 2008/0281154 corresponding to Japanese Patent No. 3583731).
Also, there is studied a method for measuring information of blood flowing through a blood vessel, for example, an oxygen saturation level of hemoglobin, a blood flow, and the like (refer to Japanese Patent Laid-Open Publication No. 06-315477). In this method, the blood vessel is extracted from the endoscopic image captured under the narrow band light, and the blood information is obtained from an image signal. This method uses the illumination light of each wavelength band of 300 to 400 nm, in the vicinity of 400 nm, 400 to 500 nm, 500 to 600 nm, 450 to 850 nm, and the like. Taking the case of measuring the oxygen saturation level of the hemoglobin as an example, an optimal wavelength band is chosen from the five wavelength bands in accordance with the internal body portion to be examined. In this chosen wavelength band, two wavelengths are used as a wavelength set, including a measurement wavelength at which absorbance much varies with the oxygen saturation level of the hemoglobin and a reference wavelength at which the absorbance hardly varies therewith. Two types of light having different wavelengths are applied to the internal body portion in succession. Then, an image signal obtained under the light of the measurement wavelength is corrected using an image signal obtained under the light of the reference wavelength, in order to calculate the oxygen saturation level of the blood through the blood vessel.
By the way, how deep light penetrates into human tissue depends on a wavelength band of the light. Taking advantage of this property, the depth of the lesion such as cancer can be inspected. More specifically, switching among the wavelength sets makes it possible to measure the oxygen saturation level of the blood flowing through the blood vessels in different layers of mucosa from a mucosal surface to a submucosal layer. This allows inspection of the depth or spread of the cancer.
The Japanese Patent Laid-Open Publication No. 06-315477 does not specifically disclose switching timing of the wavelength sets. If the switching is performed manually, complicated operation is required. Furthermore, since the manual switching tends to take long time, the observed portion happens to move. In the case of automatic switching, on the other hand, the switching is performed even during observation of a normal portion where no switching is required. Thereby, the oxygen saturation level is calculated in vain.